Air and Carbon
by Gene Franks
This article appeared originally in the Pure Water Occasional for October, 2010.
A little understood fact about granular carbon—any carbon used in filters, as a matter of fact—is that it contains a lot of air. What appears to be a pile of dry granules is actually the hiding place for countless tiny pockets of air. According to an article in Water Quality Products Magazine: “In a typical bed of dry activated carbon, the carbon skeleton only occupies 20 percent of the bed. The remainder is air.”
About half the trapped air is in the voids between the granules of the carbon. The other half is in the pores of the granules. Carbon granules are shot full of acres of many tiny crevices. These nooks and crannies are the very thing that gives carbon so much surface area and makes it such an effective adsorbent. A carbon particle only around 0.1 mm wide has a surface area of several square metres. In more dramatic terms, an EPA document states that GAC has an adsorption surface area from around 73 to 112 acres per pound!
Owners of even small carbon filters know that when water encounters carbon for the first time a lot of hissing and spluttering occurs. This is the sound of trapped air that is escaping from within the carbon.
In larger carbon beds used in tank-style filters it is often advantageous to allow a long soak—from overnight to several days, depending on the bed size and water temperature. This is because large air pockets can make the filter perform poorly. The filter, in fact, will not perform normally until all the air is gone. In some very large systems technicians resort to introducing heated water to speed the process up. This isn’t recommended for homeowners.In small filters, trapped air is often just an an aesthetic inconvenience, but it can sometimes cause “vapor locks” in undersink filters and reverse osmosis units. This condition can be relieved by simply opening a filter canister to allow the trapped air to escape. In RO units, most prefilter air escapes through the drain line of the membrane housing (that’s the hissing you hear when you start a new or a newly-serviced unit), and most postfilter air is expelled through the faucet. Rinsing the unit well usually gets rid of excess air quickly.
A vigorous backwash of up to 30 or 40 minutes can serve three purposes in new backwashing filters: it rids the carbon of fines (carbon dust), it resettles the bed so that smaller granules work their way to the top, and it clears out air pockets.
The best policy for starting non-backwashing In/Out-type filters is probably a very long soak before the unit goes into service. It could take up to 48 hours to get all the air out at ambient temperature, but the longer soak you can give the carbon before putting the filter in service the less air you’ll get into your home’s water pipes. Always open a downstream faucet to allow air to escape.
FYI: in industrial applications, air-release time can be cut to 3 to 4 hours by using 212 degree F. water. At 1800 degrees, air expulsion is instantaneous.